Optical access networks may be employed to deliver a wide variety of services, such as fiber to the home (FTTH), fiber to the building (FTTB), enterprise connectivity, business connectivity, and mobile back-haul and front-haul for fourth generation (4G) and/or next generation wireless communication. Continuous demands for higher network capacities and greater distance coverage pose challenges in current and future optical access network designs. For example, the effect of fiber transmission impairments may become more severe as the optical transmission speed and the transmission distance increase. Fiber transmission impairments may include chromatic dispersion (CD), polarization mode dispersion (PMD), phase noise, and non-linear effects. However, CD may be one of the most performance limiting factors, especially for high-speed transmissions at long distances.
CD may cause different spectral components (e.g., wavelengths) in an optical signal to travel through an optical fiber at different speeds and arrive at a receiver at different time instants (e.g., with different delays), and thus may temporally broaden the optical pulses that carry the data and lead to inter-symbol interference (ISI). Some systems may compensate CD in a fiber by employing another fiber of opposite-sign dispersion, but may be at the expense of increased loss, complexity, and cost. Recent advances in high-speed analog-to-digital converters (ADCs), high-speed digital-to-analog converters (DACs), and high performance digital signal processors (DSPs) have enabled fiber-optic impairments to be compensated digitally by DSPs.
The dispersion effect experienced by an optical signal when traveling through a given optical fiber link may be compensated through dispersion pre-compensation at a transmitter by an amount that is nominally the opposite of the fiber link dispersion. However, in typical optical access networks, the transmitter in an optical line terminal (OLT) may send time-division-multiplexed (TDM) signal blocks to multiple optical network units (ONUs), which may be located at different distances away from the OLT. Thus, the TDM blocks that are destined to different ONUs may experience different fiber link dispersions, and thus the OLT may not employ the same fiber dispersion pre-compensation for all the TDM blocks. In addition, direct-detection (DD) may be commonly employed at the ONUs' receivers, thus the OLT's transmitter may require an optical receiver-specific dispersion pre-compensation scheme that is suitable for DD receivers.